Nonemitting electron tube grid



Patented Nov. 27, 1951 NONEM'ITTING ELEGTRON TUBE GRID Irvin Levin, Asbury Park, N. .L, assignor to the United States of America as represented by the Secretary of War No Drawing. Application December 20, 1944, Serial No. 569,069

(Granted under the act of March 3, 1883, as

2 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

My invention relates to the coating of metals and, more particularly, to the coating of grid wires such as used in vacuum tubes.

In a thermionic vacuum tube containing an electron source as a cathode, one or more grids of a bare metal, and a suitable anode, the hot grid or grids will thermionically emit electrons when material from the cathode evaporates onto it. The theoretical reasons for this may be summarized, as follows: certain substances, such as thoria, enhance the ability of an electron to escape to the outside of the heated cathode metal, such as tungsten. These substances will also enhance the electron escaping ability of the hot grid metal when deposited upon the grid so that the grid will emit. For this reason it has been found that a mossy or spongy refractory metallic coating on the grid surface will prevent thermionic emission because the contaminating material from the cathode probably can migrate below the top grid surface and thus be removed from the electric fields among the tube elements.

Grid contamination means the molecular migration of thorium, barium, strontium or other materials which enhance the electron escaping ability of metallic surfaces from the vacuum tube cathode to the grid, causing said grid to primarily emit electrons. The grid will, therefore, act as an electron-emitter and lose its controlling action. Previous methods of cleaning the grid to restore its controlling action called for an extra current source to electrically heat the grid wires when contamination effects were felt, thereby evaporating off the contaminating materials.

It has been found that when a mossy layer of metal is placed over a grid wire, the heat emissivity of the wire becomes large. This mossy layer may be a spongy state of the metal or even a properly sintered or fritted metallic structure. so long as the contaminating material from the cathode can be absorbed into the mossy surface and diffuse into the mossy sheath. The large heat emissivity probably causes a large thermal gradient which, in turn, causes the contaminating material to diffuse toward the inner part of the sheath. The thermal gradient is set up by the outer surface of the grid sheath being heated by radiation from the anode and the cathode. The internal portion of the coating is cooled somewhat by the base metal underneath, thereby setting up a heat differential, or it may be a physicaamended April 30, 1928; 370 0. G. 757) 2 chemical combination between contaminating material and the mossy coating.

The actual mossy metallic grid coating is prepared from metals which do not melt or fuse at the maximum operating temperature of the grid. Common metals of this type are tantalum, zirconium, molybdenum and tungsten, although the six metals of the platinum family have been used.

The object of the invention is to provide a nonemitting electron tube grid.

The application of these coatings is: done by spraying or electro-phoretically depositing the powdered oxides or other compounds of tantalum, zirconium, molybdenum, tungsten and the six metals of the platinum family, which compounds are easily reduced to the metallic state in a vacuum at elevated temperatures.

The following examples illustrate my invention:

(1) Electrophoretic dep0siti0n.-2 grams of ignited tantalum pentoxide are dispersed in 100 c. c. of acetone. The grid to be coated is rendered positive from 20 to 100 volts because the colloidal tantalum pentoxide micelle is negatively charged. No stirring is performed during electrophoresis. The coated grid is then placed in a tantalum or molybdenum container under vacuum or reduced pressure and flash-heated by induction to 1600 C. for as short a time as possible. All traces of oxygen must be removed from the coating as well as the pumping system. The resulting grid is then mounted in the tube.

(2) Spraying.The following spray mixture has been found adequate for depositing tantalum pentoxide onto grid wires:

Amyl acetate c.c. 800 Ethyl alcohol c. c. 200 Nitrocellulose (about dry) g. 20 Tantalum pentoxide g. .300

The mixture must be thoroughly homogeneous before spraying. The coating is applied, allowed to dry, and then reduced in a vacuum at elevated temperatures.

Having thus described my invention, I desire to secure by Letters Patent and claim:

1. An electron tube having a cathode coated with an evaporable material for increasing the electron emission therefrom, a control grid which becomes heated during the operation of said tube and which tends to become coated with said evaporable material during tube operation, and means for preventing said control grid, when so heated, from acting as an emitter of primary electrons due to the deposit of said evaporable material, said means comprising a mossy, continuous, metallic oxide-tree covering on said control grid of a metal selected from the refractory group comprising tantalum, zirconium, molybdenum, tungsten and the metals or the platinum group, whereby any evaporable material reaching said control grid is absorbed into said mossy covering and primary electron emission from said control grid is prevented.

2. An electron tube having a cathode coated with an evaporable material for increasing the electron emission therefrom, a control grid which becomes heated during the operation of said tube and which tends to become coated with said evaporable material during tube operation, and means for preventing said control grid, when so heated, from acting as an emitter of primary electrons due to the deposit of said evaporable material, said means comprising a mossy, continuous, metallic oxide-free covering on said control grid of a metal of the platinum group, whereby any evaporable material reaching said control grid is absorbed into said mossy covering and primary electron emission from said control grid is prevented.

IRVIN LEVIN.

REFERENCES crrEn The following references are of record in the fil of this patent:

UNITED STATES PATENTS Number Name Date 1,370,967 Hommel Mar. 8, 1921 1,639,698 Holbom Aug. 23, 1927 1,658,712 Fonda Feb. 7, 1928 1,794,315 Mullaney Feb. 24, 1931 1,880,937 Elsey Oct. 4, 1932 1,953,254 Parker Apr. 3, 1934 1,954,353 Ernst Apr. 10, 1934 2,263,164 Dailey Nov. 18, 1941 2,417,730 Becker Mar. 18, 1947 FOREIGN PATENTS Number Country Date 475,183 Great Britain Feb. 12, 1936 

